Polyamide substrates, such as carpeting, upholstery fabric and the like, are subject to staining by a variety of agents, e.g., foods and beverages. An especially troublesome staining agent is FD&C Red Dye No. 40, commonly found in soft drink preparations. To deal with such staining problems, it has been proposed that only apply to the substrate a composition containing a sulfonated phenol-formaldehyde condensation product. For example, Blyth and Ucci, in U.S. Pat. No. 4,592,940, describe the preparation of stain-resistant nylon carpet by immersing the carpent in an aqueous solution of a sulfonated condensation polymer wherein at least 40% of the polymer units contain --SO.sub.3 X radicals and at least 40% of the polymer units contain sulfone linkages. On the other hand, in U.S. Pat. No. 4,501,591, Ucci and Blyth disclose continuously dyeing polyamide carpet fibers in the presence of an alkali metal m-silicate and a sulfonated phenol-formaldehyde condensation product so as to impart stain-resistance to the dyed carpet.
However, sulfonated phenol-formaldehyde condensation products are themselves subject to discoloration; commonly they turn yellow. W. H. Hemmpel, in a Mar. 19, 1982 article in America's Textiles entitled Reversible Yellowing Not Finisher's Fault, attributes yellowing to exposure of a phenol-based finish to nitrogen oxides and/or ultraviolet radiation. Liss and Beck, in U.S. patent application Ser. No. 124,866, filed Nov. 23, 1987, prepared modified condensation products having a reduced tendency to yellow by acylating or etherifying some of the free hydroxyl groups of the sulfonated phenol-formaldehyde condensation product, separating from it lower molecular weight materials which contribute to yellowing and which are soluble in water at a pH between about 4 and 8, and dissolving the acylated or etherified condensation product in a hydroxy-containing solvent, such as ethylene glycol. That approach works well in removing color-formers; however, such solvents can place an increased biological oxygen demand on water systems.